Bearings play a crucial role in the modern industrial world. Bearings are important factors to determine the performance and accuracy of products. In particular, bearings supporting the high-speed rotation of a product at high temperature or under high load conditions need to maintain a certain level of performance even under extreme conditions. Bearings can fall into broad categories: slide bearings and rolling bearings. Thus, the slide bearings or the rolling bearings can be selected as one more stable than the other for such as use condition, and purpose. In some products, the both types of bearings may be alternatively used at one and the same portion of one and the same product.
For example, both floating metal bearings and ball bearings are alternatively used as bearings for turbine shafts in exhaust gas turbine superchargers for use in internal combustion engines of automobiles. In comparison, the ball bearings have more advantages in performance related to such as friction. However, the ball bearing turbines basically need a larger amount of oil. Thus, if the amount of oil is not suppressed by providing an orifice to a banjo bolt of an oil line or other parts, several problems occur due to excessive oil. In addition, the rotation speed of the turbines may exceed 200,000 rpm. The turbines are directly subject to high-temperature exhaust gas (800 to 900° C.). Thus, the oil is more prone to deteriorate as compared to the case where the oil is exposed to natural intake air. In some cases, therefore, the floating metal bearings may be selected instead of the ball bearings requiring severe oil management. In addition, the floating metal bearings are lower in manufacturing cost and running cost.
Exhaust gas turbine turbochargers allow a significant output improvement with a small displacement. The exhaust gas turbine turbochargers are in particular compatible with diesel engines. Thus, just like hybrid cars and EVs, attention is now given to eco-cars with a combination of a smaller-displacement gasoline engine and a turbocharger, clean diesel cars with a combination of an emission-reduced diesel engine and a turbocharger, and the like.
Therefore, under social conditions related to environments or resources, it is strongly desired in the future to improve the floating metal bearings having few problems with such as cost and oil management in performance so as to have friction characteristics and durability on a par with the ball bearings. However, the slide bearings still have a major problem of being more prone to cause noise as compared to the ball bearings. The issue of eliminating noise and the like from an object sliding at a high speed cannot be solved immediately. To counter the foregoing problem, significant improvements have been made in the technical field of slide bearings by technical developments of floating and the like. However, these improvements still cannot be said to be sufficient.
Various techniques have been suggested to meet the social demand. For example, there is a publicly-known technique for the bearing structure of a turbocharger that rotatably supports a rotary shaft connecting a pair of wheels by a fluid bearing, the bearing structure suppressing the rotation speed of a floating metal. See, for example, Japanese Patent Application Publication No. JP-A 2008-190498 (“JP '498”). The technique allows the bearing structure to suppress occurrence of noise resulting from whirl vibration of the rotary shaft. Similarly, there is a publicly-known technique for a bearing structure that rotatably supports a rotation shaft by a pair of fluid bearings, the bearing structure suppressing occurrence of noise resulting from whirl vibration of the rotation shaft. See, for example, Japanese Patent Application Publication No. JP-A 2008-111502 (“JP '502”).
The goals of the inventions disclosed in, for example, the JP '498 and the JP '502 are both to “suppress occurrence of noise,” which is similar to the goal of the present invention to “reduce noise.” However, at a fluid-lubricated bearing that generates a pressure (dynamic pressure) on a lubricating fluid film by relative sliding movement due to rotation and supports a load under the pressure, the dynamic pressure acts on the entire circumference of the floating metal bearing. Thus, the centering effect of metal due to the dynamic pressure is weaker than that in the present invention. This is because the cross section area of the flow passage in which the fluid flows is small, and thus no large flow velocity difference is generated even when the stopped shaft begins to rotate. In addition, the foregoing conventional techniques have complicate structures and still have cost-related problems. Specifically, oil is one of the structures for suppressing the rotation of a bush floating metal to suppress whirl vibration, and another one of the structures is an outer bush. These are separated from each other in an axial direction around which the floating metal in the rotary shaft is rotated.
As another conventional technique, there is a publicly-known technique for a bearing mechanism in a turbocharger. See, for example, Japanese Utility Model No. JP-U 2517541 (“JP '541). In the bearing mechanism, a turbine shaft is supported in a bearing housing via a radial bearing, a thrust bearing, and bushes. At the turbine rotor side, the shaft is supported by one radial bearing held in the bearing housing. At the compressor side, the first bush, the thrust bearing fixed to the bearing housing, and the second bush are sandwiched in this order between the shaft and the compressor rotor. The first bush has a cylinder portion extending between the outer peripheral surface of the shaft and the inner peripheral surface of the thrust bearing. Also in the bearing mechanism, a semicircular oil groove is formed in an inner peripheral surface facing the cylindrical portion of the thrust bearing. Lubricating oil is supplied to the semicircular oil groove from an oil hole in the bearing housing through an oil hole provided in the thrust bearing.
However, the foregoing turbocharger bearing structure includes a large number of components (such as the thrust bearing, the radial bearing, the plurality of bushes, and an air pressurization mechanism). In addition, the bearing housing accommodating these components may be difficult to be provided in terms of reduction in space and cost.
Further, the foregoing patent and utility model documents do not suggest or describe any configuration for eliminating noise.
Therefore, the foregoing conventional techniques have not solved the issue of meeting the social demand expected for floating metal bearings.
The inventor of the invention of the subject application has reviewed various machining conditions such as surface roughness or roundness. The inventor then has revealed that, even if high-accuracy roundness characteristics or surface roughness characteristics are improved, it is difficult to suppress occurrence of noise, that is, occurrence of vibrations resulting in the noise. In light of the foregoing background, the inventor has repeatedly carried out experiments with changes in roundness, under the assumption that the roundness of the inner surface of a shaft bearing hole can be polygonally altered to suppress occurrence of vibrations. As a result, the present inventors have completed the present invention.